In providing electrical connections to silicon for microelectronics applications, it is frequently desired to provide a conductive interface between a metallization layer, such as aluminum (Al), and the portion of silicon to be electrically connected to the aluminum to prevent direct contact of the aluminum with the silicon. If aluminum were to be directly deposited onto the silicon, various problems may arise.
One significant problem in depositing aluminum directly on silicon is that the aluminum acts as a P-type dopant, and any migration of aluminum atoms into a silicon region would dope the silicon region with P-type impurities. This is especially significant when the aluminum contacts an N-type silicon region, where the migration of P-type aluminum atoms into the silicon region results in an undesired rectifying contact.
Another problem is that the aluminum is prone to spiking through shallow silicon regions, thus causing the aluminum to contact the underlying or adjacent silicon region.
A further problem relates to the shadowing effect, where an oxide layer is etched to expose a silicon region, and the relatively high step between the top surface of the oxide and the exposed silicon results in uneven deposition of aluminum on the exposed silicon surface and on the oxide walls. This may even result in an open circuit due to insufficient aluminum being deposited on the exposed silicon or oxide walls.
With decreasing geometries, the above problems become more acute. Further, increased reaction temperatures exacerbate the above-mentioned problems, since the migration of aluminum atoms into the silicon is accelerated with increasing temperatures. Additionally, over a period of time, the aluminum atoms further migrate into the silicon causing latent defects and low reliability contacts.
Various schemes are used in the prior art to avoid the above-mentioned problems incurred by direct contact of aluminum with silicon. One way to overcome the above-mentioned problems is to saturate the aluminum during deposition with silicon (typically 0.2-1 weight % of Si to Al) to inhibit migration of the aluminum atoms. One drawback with this method is that the saturation level of aluminum is higher at higher temperatures so that when the saturated aluminum cools, a precipitate of silicon exists at the surface of the silicon substrate under the aluminum layer, resulting in an undesired P-type epitaxial layer. Additionally, the problem of shadowing effect still exists using the above method.
Another prior art method used to avoid the problems resulting from direct contact of aluminum with silicon is to form one or more interface layers over the silicon before depositing the aluminum metallization layer. This type of process is generally described in U.S. Pat. No. 3,777,364 to Schinella et al., although numerous other patents describe similar processes In this prior art method, a refractory metal, such as tungsten, molybdenum, palladium, platinum, or tantalum, is deposited and reacted with the exposed silicon (or a polysilicon layer) to form a silicide layer. The top portion of the deposited refractory metal, which has not reacted with the silicon, may then be removed. The aluminum metallization layer is then deposited. The resulting silicide layer between the aluminum and silicon acts as a barrier to the aluminum atoms, preventing migration of the aluminum into the silicon, and provides a low resistivity contact between the aluminum and the silicon. Additionally, this type of process slightly reduces the step height (e.g., by 1200.ANG.) during deposition of the aluminum metal layer.
A problem with the above teaching of forming a silicide layer is that, when forming, for example, tungsten silicide as the interface layer between aluminum and silicon using a chemical vapor deposition (CVD) process and tungsten hexafluoride (WF.sub.6) as the reactant gas, the high temperatures involved in the CVD process cause the hot CVD chamber walls to react with the WF.sub.6 gas. This results in a lowering of the deposition rate of the tungsten onto the surface of the wafer.
Although sputtering tungsten onto the surface of the wafer does not incur the problem of hot walls of a chamber reacting with the metallic fluoride gas, sputtering refractory metal films is subject to the following limitations: (1) such films are frequently highly stressed and crack; (2) such films are characterized by relatively poor step coverage; (3) such films are relatively costly to produce; (4) only a relatively small number of wafers can be processed in a given amount of time; and (5) sputtering equipment is relatively expensive to purchase and operate.
Thus, using a hot-wall CVD process to deposit a refractory metal onto the silicon results in the deposition rate being limited, and the problem with step height is not avoided to the extent desirable.
U.S. Pat. No. 4,794,019 to Miller describes a tungsten deposition process using a hot-wall CVD chamber and mentions that the deposition rate of the tungsten decreases with time to reach a stable value of 25 .ANG./min at a thickness of about 3000 .ANG. (col. 3, lines 31-33). Miller teaches a CVD process temperature on the order of 300.degree. C. to deposit the tungsten on the surface of the silicon wafer. Since tungsten nucleates well to silicon but not to SiO.sub.2 at this temperature, this process is used to selectively deposit the tungsten on the silicon and not on SiO.sub.2. By using this method, however, an increase in the thickness of the refractory metal over approximately 3000 .ANG. is relatively time consuming and expensive.
One way used to solve the problem of low deposition rates due to the hot walls of a chamber reacting with the metallic fluoride gas is to heat the wafers locally using direct irradiation of the wafer by means of a quartz halogen lamp or some other source of radiation. This method of heating the wafer, however, results in different refractory metal deposition rates on N and P type silicon regions due to the different emissivities of these two regions when heated by a radiation source. This uneven deposition increases the difficulty of efficiently depositing a metal interconnect layer providing good electrical contact to both the N and P type silicon regions.
What would be desirable in the industry is a method of depositing a refractory metal as a barrier layer between aluminum and silicon at a relatively high deposition rate so as to reduce step height to a desirable amount, and which would result in an even deposition of refractory metal over both exposed N and P type silicon regions.